Blu-ray Disc (BD) has already been put to practical use as a recording medium for high definition television image, and further BD is spreading increasingly due to the standard integration. Recently, a super high definition television system is proposed as a next generation image technology. Accordingly, there is a growing demand for a further increase in the capacity of an optical disc as a recording medium. As a method for increasing the capacity, a multilayer technique and a high-density technique may be used. The multilayer technique can double or treble the capacity by providing two or more information layers. On the other hand, a technique using near-field light is proposed in the high-density technique.
An optical system combining a collective lens and a solid immersion lens (SIL) recently has attracted attention as an optical beam focusing device using near-field light. A higher numerical aperture can be achieved by this combination than the NA (numerical aperture) of a collective lens. By increasing the numerical aperture of an optical system, it is possible to decrease the diameter of a spot, and thus high density recording is feasible. For example, the recording capacity of a BD is 25 GB per information layer, but use of an SIL for its optical system allows a recording capacity of 60 GB or more per information layer to be achieved.
In an optical system using an SIL, the distance between the SIL and an optical disc surface is required to be extremely shortened because it is necessary that a laser beam leaking from the emitting surface of the SIL be allowed to enter the optical disc surface by the generated near-field light. For such a reason, in an optical system of a BD, the distance between an objective lens and an optical disc surface is about 0.3 mm, whereas in an optical system thereof using an SIL, the distance between the emitting surface of the SIL and an optical disc surface is about 50 nm or less.
Further, in the case of a medium for recording/reproducing information using an SIL, it also is necessary to shorten the distance between the SIL and a recording layer. In a DVD (Digital Versatile Disc), a 0.6 mm thick substrate is provided on the laser beam incident side, while in the case of a BD, a 100 μm thick transparent layer is provided on the laser beam incident side. However, in a medium for recording/reproducing information using an optical system using an SIL, a transparent layer to be provided on the laser beam incident side has a thickness of 5 μm or less. In addition, in the case of a multilayer medium including a plurality of information layers (see, for example, Patent Literature 1), an interlayer having the function of separating between information layers is provided. Whereas the thickness of an interlayer is 25 μm in the case of a two-layer BD, the thickness of an interlayer is 5 μm or less in the case of a multilayer medium for recording/reproducing information using an SIL (see, for example, Non Patent Literature 1).
As seen from above, Patent Literature 1 and Non Patent Literature 1 each disclose a structure of a single layer or multilayer medium for recording/reproducing information using an optical system using an SIL.